Linear mechanical actuators of shape memory alloy (SMA) compositions have been used in a variety of devices, many of which have been conceived for use on automotive vehicles. For example, U.S. Pat. Nos. 7,607,717, 7,686,382, and 8,188,757, and Patent Application Publications 2012/0174573 and 2012/0184195, each assigned to the assignee of this invention, describe and illustrate a number of on-vehicle devices using linear mechanical actuators formed of shape memory alloys such as nickel-titanium based alloys. The portions of these patent disclosures pertaining to compositions, shapes, and functions of shape memory alloy, mechanical actuators for devices are incorporated herein by reference. These documents describe and illustrate that movable air flow dams, air flow spoilers, and baffle controllers within Heating Ventilation Air Conditioning outlet housings are examples of on-vehicle devices that may be set in motion by linear SMA actuators.
Often the SMA actuator is in the shape of a wire (or band, strip, cable, or other generally linear shape) and a change in the length of the actuator is used to move or otherwise activate a movable member or element of the device. The change in length of the actuator is typically achieved by exploiting the metallurgical and mechanical properties of a selected alloy composition. A desired remembered-length characteristic is formed in the linear actuator at a suitable elevated temperature at which the metallurgical alloy composition is in its austenitic phase. In the next manufacturing step the intended linear actuator is cooled to a lower temperature region at which it transforms to it martensite phase. This lower temperature region is preferably the ambient temperature region in which the device is to be operated. When the linear actuator material is in its martensite phase it displays an approximately 2.5 times decrease in stiffness, and it is stretched (sometimes termed “pseudo plastically deformed”) to a longer length. The longer length of the linear actuator material is utilized as its “ready” actuator length. Then, upon a need for an activation function, the actuator is heated to re-transform it to its austenite phase. With such heating and metallurgical phase transformation, the wire experiences an approximately 2.5 times increase in stiffness, undergoes change in its electrical resistance, and it shrinks to its remembered length and, thereby, moves and repositions the movable part of the device in which it is placed.
Thus, in many uses, the metal alloy composition of the wire actuator is prepared so that the wire has a predetermined length at an ambient temperature for the use of the device. This initial length of the actuator is placed in the device and retains that length until an element of the device is to be moved or otherwise actuated. Upon a suitable signal for actuation of the device, the wire is then heated, such as by electrical resistance heating. As the wire is heated it shrinks in length (e.g., five to eight percent of its ambient temperature length) to move some part of the device. The heating and phase transformation is typically accomplished in a relatively short period of seconds or minutes depending on the ambient temperature, the size of the linear actuator, and the current flow. But some heating of the actuator is continued while the device is in its actuated condition so that the actuator is maintained in the shortened, remembered length of its austenitic phase. When de-activation of the device is signaled, heating is stopped and the actuator is cooled back to its martensite phase, often by heat transfer to the ambient environment. As the SMA actuator is cooled, it softens and is stretched with a complementary, attached spring in its device, to strain the SMA wire to its intended length for the next actuation of the device in which it is employed. The movable component of the device is returned to its rest position as the SMA wire is being elongated. The moving component may be pulled against a stop in the device to assure that the SMA actuator is returned to its original length.
While the function of such linear SMA actuators is relatively simple, a typical actuator is usually expected to experience many repeated metallurgical phase transformations as well as repeated length-stretching deformations in order to serve its purpose in its air dam or other on-vehicle device. These repeated phase transformations and deformations of the actuator may result in unwanted changes in its ambient temperature length and its capacity to undergo full work-producing phase transformations. Further, there may be unwanted mechanical changes in the other elements of the device in which the actuator is employed. There is a need for low cost and relatively inexpensive diagnostic procedures to detect problems and deterioration in the performance in the operation of the SMA actuator and related problems arising in associated elements of the actuated device which affect the viability or “health” of the actuator and device. Often such SMA actuated devices may be employed in difficult-to-reach locations on automotive vehicles, and often there is a need to inform the operator of the vehicle of such problems.